The present invention is directed primarily to apparatus commonly referred to as wood hoggers, or reducers. Typically these comprise apparatus for reducing or comminuting wooden material into smaller sized pieces. Typically the raw feed material is waste pieces of logs, timber, or other wood based material which can then be converted into a combustible fuel source suitable for use in boilers, and the like, and which is commonly known as hog fuel. In practice, however, operators have been used to reduce a variety of different types of materials, such as will be discussed later.
‘Hog fuel’ is a combustible solid fuel often used in boilers and which is primarily made up of wood residue from sawmills, logging operations, and various wood off-cuts. Hog fuel typically varies from chipped wood as it commonly may comprise timber of a variety of sizes rather than substantially uniform chips of wood. Typically hog fuel may comprise pieces of wood typically 50 mm×50 mm×120 mm down to sawdust. It is commonly made from product which would otherwise be dumped, and may contain a significant amount of foreign materials such as dirt, and stones, depending on the source of the material and how well the material is screened prior to processing.
The machines used to prepare hog fuel are commonly known as ‘hoggers’ and may also be called reducers, the latter being a term more commonly used on some types of design, or as a preferred name in different regions. For the purpose of the prior art description we shall use the term ‘hogger’ to also include ‘reducer’ unless a distinction is otherwise made.
Quite a number of different sizes of hogger exist, and in most case rely on rotating reducing discs or drums with chipping or cutting teeth or features. In most instances these are used in conjunction with a grate which screens reduced material suitable for hog fuel away from product which has not yet been sufficiently reduced. Most designs have a number of flaws or problems associated with them, which at least partially counts for the presence of a number of substantially different standard designs available on the market.
A common problem that affects most designs is energy consumption. The process of reducing timber product into smaller reduced pieces suitable for hog fuel consumes a significant amount of energy, and consideration needs to be given to the economy of the process to ensure that the energy expended does not exceed the resulting energy which can be obtained from what is essentially a low-grade fuel. While this can be partially offset by the fact that hogging is also effectively removing a waste product, by recycling, the economy of the process remains very important. In a number of instances the specific design of the hogging apparatus has a bearing on energy efficiency. These problems will be detailed more fully below.
In the art a variety of different types of hogging apparatus are known, each of which have a number of disadvantages associated with them which affect their efficiency, safety, or ability to process a wide range of material. The common types of hogging apparatus known to the inventor will now be described.
Traditionally, devices known in the United States as reducers were used. One design is illustrated in FIGS. 1a and 1b. This basically comprises a horizontally rotating drum (1) with a plurality of teeth (2) distributed thereabout. Shielding the majority of the outer face of the drum is a screen grate (3) which allows processed material of suitably small size to pass therethrough, as well as acting as a safety shield. Positioned in proximity to the teeth of the rotating drum are one or more anvils (4) whose distance of separation from the teeth (2) largely determines the size of the resulting material. As raw material is fed into the apparatus (generally indicated in direction by arrow (5)), the teeth basically mash wood against the anvil and break it down into a reduced size reflecting a distance of the anvil from the teeth. This material is largely carried through an exit via the grate (3).
However, this design suffers from high energy usage and substantial wear on the teeth and anvil. More significantly however, materials such as steel, metal, and very hard rocks can basically jam the system. A typical result is either shearing of the teeth from the drum, or damage to other components. This catastrophic destruction of components can be expensive and time consuming to fix. Given that a lot of forestry material may contain steel pins from logging trucks, or other hard foreign material, this type of apparatus either needs to be precluded from processing such materials, or the raw material subjected to additional time consuming, and potentially expensive, cleaning steps. Given that the hog fuel is a low grade and inexpensive fuel, expensive cleaning steps cannot normally be justified. The high energy usage of this type of apparatus also reduces any profit margin which may be gained from the production of hog fuel.
Additionally, as timber may come from a variety of sources including demolition materials, this type of apparatus also suffers another type of problem related to stringy material. When material such as flax, long fibrous leaves, or carpet are fed into the system, the grate becomes quickly clogged and the machine stalls as material then becomes wound around the drum. Such material can stall this type of apparatus in as little as three seconds, and may require several hours for it to be pulled apart and fixed.
In another type of apparatus a horizontally rotating drum is placed at the bottom of a shallow pan. The drum occupies an aperture in the base of the pan, and the material is substantially gravity fed towards the rotating drum. Additionally or alternatively the pan itself may also rotate. This arrangement is shown generally in FIGS. 2a-c. 
Processed material exits through a grate (20) covering the portion of the drum (21) positioned below the pan (22). Material too large to pass through the grate (20) is generally returned to the pan and eventually comes once again into contact with the rotating drum. In this type of apparatus there is no anvil, with the drum essentially nibbling at the material within the pan from the bottom. However this type of apparatus has a tendency to fire projectiles (such as foreign material) upwards, thus introducing safety issues. It will effectively eat and nibble away at just about anything though is even more susceptible to steel, and also suffers problems with stringy materials. There is also a very high energy usage for this type of apparatus.
Another variation is a tub-type hogger where a horizontal disc is positioned near the bottom. This disc has teeth about its outer peripheral and also on its top, and sometimes bottom, surfaces. This is generally shown in FIG. 3a-b. In this type of hogger either the disc (30) can rotate, and/or the tub (3) may rotate around the disc, ensuring relative motion of the raw material which is held within the tub with respect to the disc. The bottom of the tub is essentially a grate (32) which allows processed material of sufficiently reduced size to fall free from the bottom. While this design is less likely to fire high speed projectiles, and is also more tolerant of hard materials such as metal and steel etc, it does suffer from high wear as the raw material is in contact with the rotor all of the time. This continuous contact also reduces energy efficiency and the apparatus requires significant amount of power in order to operate. Further, it has been found that the screening in this type of apparatus can be inconsistent and it depends largely on the amount of material which is in the tub at any one time—for instance, a low load can allow big pieces to pass through while a high load (i.e. lots of material in the tub) may only let much smaller material out. In this type of apparatus energy efficiency is very much dependent on maintaining optimum fill rates. Overfilling tends to create a situation where little screening occurs, or only in the vicinity of the rotating disc. In such cases there is also a tendency for already reduced material to keep being reintroduced to the disc, resulting in poor energy efficiency and high tooth wear in terms of throughput. The design is also susceptible to green plant material, which tends to clog the screen except in close proximity to the rotating disc. The resulting significant reduction in screen area seriously effects throughput and efficiency.
The fourth type of hogger in usage, though which is relatively uncommon, is like a chipper. It has a rotating disc in which bulk unprocessed material is fed to interact with the top rotating surface. This type of apparatus is very efficient, though is restricted to only processing full stems of material. There is also very high blade wear and it is also very susceptible to steel and metal. This type of hogger is typically restricted only for uses where trees are specially grown for fuel, and introduces the added requirement that felled logs must be handled very carefully to avoid picking up rocks or other foreign material which could damage the apparatus.
As can be seen from the above description, quite a range of approaches have been proposed for producing hog fuel. However each one of these designs suffers from one or more disadvantages which are commonly related to safety issues, durability and maintenance of the apparatus, and efficiency of production. From an economical perspective, given the low value of hog fuel, there is a requirement that hog fuel must be produced efficiently and at low cost, and that maintenance and repair costs cannot be to the extent they substantially impact on the profitability of the process.
Therefore there is a need for an alternative process which can provide an efficient means for producing hog fuel, and which ideally is able to process a wide range of materials.
It has also been previously mentioned that hoggers have also been used to process a variety of other materials, and appear to be advancing into a role as a recycling machine. The type of materials being reduced depends very much on hogger design, though known uses include the reduction of: demolition timber, demolition masonry, mild steel scrap (thin sections), soft metal scrap, recycled cans, recycled glass, recycled plastic, soft rocks, occasionally hard rocks, recycled roading and asphalt, old tyres, green garden waste, etc. As can be appreciated from the above description, a majority of these materials would be fatal to a number of known hogger and reducer designs. These materials can also exacerbate many of the problems previously mentioned and thus potentially useful and valuable recycling apparatus is often precluded from widespread use in these additional roles.
It is therefore an object of the present invention to propose apparatus which represents an alternative and efficient means of producing hog fuel from a range of wood based materials.
It is an object of the present invention to consider an improved design which lends itself to the processing of materials other than typical forestry and timber materials traditionally used in hoggers.
It is an alternative object of the present invention to provide apparatus which addresses one or more of the problems affecting the prior art.
At the very least it is an object of the present invention to provide the public with a useful choice.
Aspects of the present invention will be described by way of example only and with reference to the ensuing description.